Biphase modulation is important for many electronic products which require digital coding of radio frequency signals, such as PN coded signals. Biphase modulation is often required in products constrained with small size, low power draw and robust operation in a broad variety of environments. Examples of such applications include munitions fuzes, radar apparatus and communications systems.
Conventional biphase modulators suffer from lack of flexibility in choosing modulation angles and are typically restricted to 0 degrees and 180 degrees modulation angles. Multiphase modulation schema (e.g. QAM) typically provide phase angles of integer submultiples of 180 degrees (e.g., 45 degrees, 90 degrees, etc.). It is especially uneconomical and impractical to provide close amplitude balance over significant bandwidth between signal portions having different modulation angles, particularly when modulation angles other than submultiples of 180 degrees are required.
Phase shifters can be either analog or digital. An analog phase shifter typically features varactor diodes whose capacitance changes as a function of bias voltage. The change in capacitance causes a change in the transmission phase of the network. The major limitation with analog phase shifters is that it is difficult to accurately switch between to voltages at high speeds (i.e. &gt;10 MHz).
Digital phase shifters are designed using switching elements such as PIN diodes or FETs. Many configurations of digital phase shifters have been demonstrated, including quadrature couplers with two ports terminated (using either FETs or PIN diodes used to provide two different angles of reflection coefficients) and PIN diode or FET switches used to select one of two different line lengths. While digital phase shifters can operate at very high speeds (&lt;10 nanoseconds), they do not provide any variation in phase shift.
What is needed is a practical, economical apparatus and accompanying method for providing biphase modulation of signals with adjustable phase angles, particularly combined with close amplitude tracking between the two phase states of the modulator. Such a biphase modulator apparatus and method should be realizable in compact form, having low power dissipation and providing robust performance over a broad range of operating conditions. Preferably, the method and apparatus would provide a means to obtain digital phase shifting with the value of phase shift adjustable by means of a control voltage. Such a device would provide the speed and accuracy of a digital phase shifter with the adjustability of an analog phase shifter, providing any desired phase shift and range of adjustment.